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    <script>
        {
            // var 声明的变量往往会越作用域 
            // let 声明的变量有严格局部作用域
            var a = 1;
            let b = 2;
        }
        console.log(a);//1
        console.log(b);//Uncaught ReferenceError: b is not defined

        // var 可以声明多次 
        // let 只能声明一次
        var m = 1;
        var m = 2;
        let n = 3;
        console.log(m);//2
        console.log(n);//Identifier 'n' has already been declared

        // var 会变量提升 
        // let 不存在变量提升
        console.log(x);// undefined
        var x = 1;
        console.log(y);//Uncaught ReferenceError: y is not defined
        let y = 2;

        // 1. 声明之后不允许改变 
        // 2. 一但声明必须初始化，否则会报错
        const c = 1;
        c = 2;//Uncaught TypeError: Assignment to constant variable.
    </script>
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